Vehicles such as automobiles generally include seat belt apparatuses that restrain an occupant in a seat that includes a seat portion in which the occupant sits and a backrest portion positioned back of the occupant. Such a seat belt apparatus includes a webbing that restrains the occupant, a seat belt retractor that retracts the webbing, a guide anchor that is disposed on a vehicle body side and guides the webbing, a belt anchor that fixes the webbing to the vehicle body side, a buckle disposed on a side surface of a seat, and a tongue disposed on the webbing. The tongue is fitted into the buckle so as to restrain the occupant with the webbing.
Such a seat belt retractor in many cases includes a spool that retracts a webbing, a base frame that accommodates the spool while allowing the spool to rotate, a spring unit that applies a retracting force to the spool, a vehicle sensor that detects rapid deceleration of the vehicle, a lock mechanism that is activated by the vehicle sensor and that causes the spool to be engaged with the base frame, and a pretensioner that removes the slack of the webbing in case of emergency such as a collision of the vehicle in accordance with a signal from the vehicle (see, for example, Japanese Unexamined Patent Application Publication No. 2014-80121 and Japanese Unexamined Patent Application Publication No. 2000-302007).